Mouse steroid sulfotransferases: substrate specificity and preliminary X-ray crystallographic analysis

Time-restricted feeding ameliorates uterine epithelial estrogen receptor α transcriptional activity at the time of embryo implantation in mice fed a high-fat diet

BACKGROUND

More than 30% of reproductive-age women are obese or overweight. Obesity and exposure to a high-fat diet (HFD) detrimentally affect endometrial development and embryo implantation. We previously reported that time-restricted feeding (TRF) improved ovarian follicular development, but whether and how TRF modulates embryo implantation are poorly understood.

OBJECTIVE

We investigated the effect of TRF on embryo implantation.

METHODS

In TRF group, mice had 10 hours of food free access from 9 pm to 7 am, and fed a normal diet or a HFD. Tail vein injection of Chicago blue dye was used to examine embryo implantation sites at day 5.5 (D5.5) of pregnancy. Serum collected at D0.5 and D4.5 of pregnancy was used to examine the level of estradiol (E2) and progesterone. Uterine estrogen receptor (ER) and progesterone receptor levels and their targeted aquaporins (AQPs) were measured. LC-MS was used to analyze bile acid (BA) composition, and primary hepatocytes were used to test the effects of BA on the expression level of SULT1E1, a key enzyme in estrogen inactivation and elimination.

RESULTS

We found that TRF prevented HFD-induced embryo loss and alleviated the defect in luminal closure on D4.5 of pregnancy. The cyclic changes of E2 level were lost in mice fed ad libitum but not in TRF mice on the HFD. The HFD increased ERα expression and transcriptional activity, which induced AQP3 and AQP5 expression on D4.5 of pregnancy. TRF prevented the negative effect of the HFD on uterine luminal closure. Furthermore, in vitro and in vivo results showed that BA suppressed estrogen degradation by activating liver SULT1E1 expression.

CONCLUSIONS

Our findings demonstrated that TRF prevented HFD-induced defects in luminal closure, thereby improving embryonic implantation, and provide novel insights into the effects of dietary intervention on obesity and associated infertility.

Nuclear receptor CAR-ERα signaling regulates the estrogen sulfotransferase gene in the liver

Estrogen sulfotransferase (SULT1E1) inactivates estrogen and regulates its metabolic homeostats. Whereas SULT1E1 is expressed low in the liver of adult mice, it is induced by phenobarbital (PB) treatment or spontaneously in diabetic livers via nuclear receptors. Utilizing constitutive active/androstane receptor (CAR) KO, estrogen receptor α (ERα KO, phosphorylation-blocked ERα S216A KI mice, it is now demonstrated that, after being activated by PB, CAR binds and recruits ERα onto the Sulte1 promoter for subsequent phosphorylation at Ser216. This phosphorylation tightens CAR interacting with ERα and to activates the promoter. Hepatic SULT1E1 mRNA levels are constitutively up-regulated in type 1 diabetic Akita mice; CAR spontaneously accumulates in the nucleus and activates the Sult1e1 promoter by recruiting phosphorylated ERα in the liver as observed with PB-induced livers. Thus, this CAR-phosphorylated ERα signaling enables these two nuclear receptors to communicate, activating the Sult1e1 gene in response to either PB or diabetes in mice. ERα phosphorylation may integrate CAR into estrogen actions, providing insights into understanding drug-hormone interactions in clinical therapy.

Cholestasis-induced bile acid elevates estrogen level via farnesoid X receptor-mediated suppression of the estrogen sulfotransferase SULT1E1

The liver is the main site of estrogen metabolism, and liver disease is usually associated with an abnormal estrogen status. However, little is known about the mechanism underlying this connection. Here, we investigated the effects of bile acid (BA)-activated farnesoid X receptor (FXR) on the metabolism of 17β-estradiol (E2) during blockage of bile flow (cholestasis). Correlations between BA levels and E2 concentrations were established in patients with cholestasis, and hepatic expression profiles of key genes involved in estrogen metabolism were investigated in both WT and FXR^-/- mice. We found that the elevated E2 level positively correlated with BA concentrations in the patients with cholestasis. We further observed that bile duct ligation (BDL) increases E2 levels in mouse serum, and this elevation effect was alleviated by deleting the FXR gene. Of note, FXR down-regulated the expression of hepatic sulfotransferase SULT1E1, the primary enzyme responsible for metabolic estrogen inactivation. At the molecular level, we found that FXR competes with the protein acetylase CREB-binding protein (CBP) for binding to the transcription factor hepatocyte nuclear factor 4α (HNF4α). This competition decreased HNF4α acetylation and nuclear retention, which, in turn, repressed HNF4α-dependent SULT1E1 gene transcription. These findings suggest that cholestasis induces BA-activated FXR activity, leading to downstream inhibition of SULT1E1 and hence impeding hepatic degradation of estrogen.

A theoretical study on the characteristics of the intermolecular interactions in the active site of human androsterone sulphotransferase: DFT calculations of NQR and NMR parameters and QTAIM analysis

A theoretical study at the level of density functional theory (DFT) was performed to characterize noncovalent intermolecular interactions, especially hydrogen bond interactions, in the active site of enzyme human androsterone sulphotransferase (SULT2A1/ADT). Geometry optimization, interaction energy, (2)H, (14)N, and (17)O electric field gradient (EFG) tensors, (1)H, (13)C, (17)O, and (15)N chemical shielding (CS) tensors, Natural Bonding Orbital (NBO) analysis, and quantum theory of atoms in molecules (QTAIM) analysis of this active site were investigated. It was found that androsterone (ADT) is able to form hydrogen bonds with residues Ser80, Ile82, and His99 of the active site. The interaction energy calculations and NBO analysis revealed that the ADT molecule forms the strongest hydrogen bond with Ser80. Results revealed that ADT interacts with the other residues through electrostatic and Van der Waals interactions. Results showed that these hydrogen bonds influence on the calculated (2)H, (14)N, and (17)O quadrupole coupling constants (QCCs), as well as (1)H, (13)C, (17)O, and (15)N CS tensors. The magnitude of the QCC and CS changes at each nucleus depends directly on its amount of contribution to the hydrogen bond interaction.

Lack of substrate inhibition in a monomeric form of human cytosolic SULT2A1

Mammalian cytosolic sulfotransferases (SULTs) frequently show substrate inhibition during the sulfation of increasing concentrations of substrates. SULT2A1, a major human liver isoform responsible for the conjugation of hydroxysteroids, bile acids and aliphatic hydroxyl groups in drugs and xenobiotics, is a homodimer and displays substrate inhibition during the conjugation of dehydroepiandrosterone (DHEA). Maltose binding protein (MBP)-SULT2A1 fusion protein, produced as an intermediate step in the purification of the SULT2A1 homodimer, elutes during size exclusion chromatography as a monomer. The initial-rate parameters (Km and Vmax) of the monomer (MBP-SULT2A1) and native SULT2A1 dimer for DHEA sulfation are extremely similar; however, the monomer is not inhibited by DHEA. Intrinsic fluorescence studies show that two DHEA molecules bind each SULT2A1 subunit, one in the catalytic site and one in an apparent allosteric site. Lack of dimerization in the MBP-SULT2A1 fusion protein decreased the Kd for binding of DHEA at the allosteric site. These results suggest that formation of the homodimer is associated with structural re-arrangements leading to increased DHEA binding at an allo-steric site that is associated with substrate inhibition.

Inhibition of EGFR autophosphorylation plays an important role in the anti-breast cancer efficacy of the dithiocarbamate derivative TM208

AIM

To investigate the effects of a novel dithiocarbamate derivative TM208 on human breast cancer cells as well as the pharmacokinetic characteristics of TM208 in human breast cancer xenograft mice.

METHODS

Human breast cancer MCF-7 and MDA-MB-231 cells were treated with TM208 or a positive control drug tamoxifen. Cell proliferation was examined using SRB and colony formation assays. Cell apoptosis was analyzed with Annexin V-FITC/PI staining assay. Protein expression was examined with Western blot, ELISA and immunohistochemical analyses. MCF-7 breast cancer xenograft nude mice were orally administered TM208 (50 or 150 mg·kg(-1)·d(-1)) or tamoxifen (50 mg·kg(-1)·d(-1)) for 18 d. On d 19, the tumors were collected for analyses. Blood samples were collected from the mice treated with the high dose of TM208, and plasma concentrations of TM208 were measured using LC-MS/MS.

RESULTS

Treatment of MCF-7 and MDA-MB-231 cells with TM208 dose-dependently inhibited the cell proliferation and colony formation in vitro (the IC50 values were 36.38 ± 3.77 and 18.13 ± 0.76 μmol/L, respectively). TM208 (20-150 μmol/L) dose-dependently induced apoptosis of both the breast cancer cells in vitro. In MCF-7 breast cancer xenograft nude mice, TM208 administration dose-dependently reduced the tumor growth, but did not result in the accumulation of TM208 or weight loss. TM208 dose-dependently inhibited the phosphorylation of EGFR and ERK1/2 in both the breast cancer cells in vitro as well as in the MCF-7 xenograft tumor.

CONCLUSION

Inhibition of EGFR autophosphorylation plays an important role in the anticancer effect of TM208 against human breast cancer.

Glucocorticoids stimulate hepatic and renal catecholamine inactivation by direct rapid induction of the dopamine sulfotransferase Sult1d1

During the stress response and metabolic fasting, glucocorticoids acting via the glucocorticoid receptor (GR) stimulate hepatic glucose production by activating specific gluconeogenic enzyme target genes. To characterize novel direct GR-regulated hepatic target genes under glucocorticoid control, we performed a whole genome gene expression microarray using dexamethasone-treated GR-null mice. Strongly induced previously characterized genes included phosphoenolpyruvate carboxykinase, serine dehydratase, tyrosine oxygenase, lipin 1, metallothionine, and cdkn1A. Novel induced genes included Ddit4, Fkbp5, Megf9, Sult1e1, and Sult1d1, and all were verified by real-time PCR. Sult1d1, a sulfotransferase, is a member of a large superfamily of detoxification enzymes and has an important role in the inactivation of endogenous dopamine-derived compounds, including the catecholamines. Treatment of primary mouse hepatocytes with dexamethasone for 6 h dramatically increased Sult1d1 mRNA levels, whereas cotreatment with RU-486, a GR antagonist, blocked induction by dexamethasone. Sult1d1 mRNA levels were also increased by dexamethasone in the kidney, a major site of Sult1d1 synthesis. Sult1d1 mRNA was localized by in situ hybridization to renal collecting ducts and was rapidly induced by glucocorticoids in renal inner medullary collecting duct (IMCD3) cells. Hepatic and renal Sult1d1 enzymatic activity was significantly induced in vivo in wild-type mice 6 h after dexamethasone treatment. Chromatin immunoprecipitation assay analysis upstream of the Sult1d1 gene promoter identified a glucocorticoid response element close to the neighboring glucocorticoid-responsive estrogen sulfotransferase Sult1e1 gene, indicating that both genes potentially share a common glucocorticoid response element. These results suggest that Sult1d1 in mice is directly induced by glucocorticoids and may attenuate elevated catecholamine activity during the stress response.

Minireview: Nuclear receptor-controlled steroid hormone synthesis and metabolism

Steroid hormones are essential in normal physiology whereas disruptions in hormonal homeostasis represent an important etiological factor for many human diseases. Steroid hormones exert most of their functions through the binding and activation of nuclear hormone receptors (NRs or NHRs), a superfamily of DNA-binding and often ligand-dependent transcription factors. In recent years, accumulating evidence has suggested that NRs can also regulate the biosynthesis and metabolism of steroid hormones. This review will focus on the recent progress in our understanding of the regulatory role of NRs in hormonal homeostasis and the implications of this regulation in physiology and diseases.

Isotope exchange at equilibrium indicates a steady state ordered kinetic mechanism for human sulfotransferase

Cytosolic sulfotransferase (SULT)-catalyzed sulfation regulates biosignaling molecular biological activities and detoxifies hydroxyl-containing xenobiotics. The universal sulfuryl group donor for SULTcatalyzed sulfation is adenosine 3'-phosphate 5'-phosphosulfate (PAPS). The reaction products are a sulfated product and adenosine 3',5'-diphosphate (PAP). Although the kinetics has been reported since the 1980s,SULT-catalyzed reaction mechanisms remain unclear. Human SULT1A1 catalyzes the sulfation of xenobiotic phenols and has very broad substrate specificity. It has been recognized as one of the most important phase II drug-metabolizing enzymes. Understanding the kinetic mechanism of this isoform is important in understanding drug metabolism and xenobiotic detoxification. In this report, we investigated the SULT1A1-catalyzed phenol sulfation mechanism. The SULT1A1-catalyzed reaction was brought to equilibrium by varying substrate (1-naphthol) and PAPS initial concentrations. Equilibrium constants were determined. Two isotopic exchanges at equilibrium ([14C]1-naphthol <=>[14C]1-naphthyl sulfate and[35S]PAPS<=>[35S]1-naphthyl sulfate) were conducted. First-order kinetics, observed for all the is otopic exchange reactions studied over the entire time scale that was monitored, indicates that the system was truly at equilibrium prior to addition of an isotopic pulse. Complete suppression of the 35S isotopic exchange rate was observed with an increase in the levels of 1-naphthol and 1-naphthyl sulfate in a constant ratio,while no suppression of the 14C exchange rate was observed with an increase in the levels of PAPS and PAP in a constant ratio. Data are consistent with a steady state ordered kinetic mechanism with PAPS and PAP binding to the free enzyme.

Glucocorticoids antagonize estrogens by glucocorticoid receptor-mediated activation of estrogen sulfotransferase

Glucocorticoids and estrogens are two classes of steroid hormones that have essential but distinct physiologic functions. Estrogens also represent a risk factor for breast cancer. It has been suggested that glucocorticoids can attenuate estrogen responses, but the mechanism by which glucocorticoids inhibit estrogenic activity is unknown. In this study, we show that activation of glucocorticoid receptor (GR) by dexamethasone (DEX) induced the expression and activity of estrogen sulfotransferase (SULT1E1 or EST), an enzyme important for the metabolic deactivation of estrogens, because sulfonated estrogens fail to activate the estrogen receptor. Treatment with DEX lowered circulating estrogens, compromised uterine estrogen responses, and inhibited estrogen-dependent breast cancer growth in vitro and in a xenograft model. We further showed that the mouse and human SULT1E1 genes are transcriptional targets of GR and deletion of Sult1e1/Est in mice abolished the DEX effect on estrogen responses. These findings have revealed a novel nuclear receptor-mediated and metabolism-based mechanism of estrogen deprivation, which may have implications in therapeutic development for breast cancers. Because glucocorticoids and estrogens are widely prescribed drugs, our results also urge caution in avoiding glucocorticoid-estrogen interactions in patients.

Estrogen Deprivation and Inhibition of Breast Cancer Growth in Vivo through Activation of the Orphan Nuclear Receptor Liver X Receptor

Estrogen plays an important role in normal physiology. It is also a risk factor for breast cancer, and antiestrogen therapies have been shown to be effective in the treatment and prevention of breast cancers. The liver is important for estrogen metabolism, and a compromised liver function has been linked to hyperestrogenism in patients. In this report, we showed that the liver X receptor (LXR) controls estrogen homeostasis by regulating the basal and inducible hepatic expression of estrogen sulfotransferase (Est, or Sult1e1), an enzyme critical for metabolic estrogen deactivation. Genetic or pharmacological activation of LXR resulted in Est induction, which in turn inhibited estrogen-dependent uterine epithelial cell proliferation and gene expression, as well as breast cancer growth in a nude mouse model of tumorigenicity. We further established that Est is a transcriptional target of LXR, and deletion of the Est gene in mice abolished the LXR effect on estrogen deprivation. Interestingly, Est regulation by LXR appeared to be liver specific, further underscoring the role of liver in estrogen metabolism. Activation of LXR failed to induce other major estrogen-metabolizing enzymes, suggesting that the LXR effect on estrogen metabolism is Est specific. In summary, our results have revealed a novel mechanism controlling estrogen homeostasis in vivo and may have implications for drug development in the treatment of breast cancer and other estrogen-related cancerous endocrine disorders.

Fluorometric assay for alcohol sulfotransferase

A sensitive fluorometric assay was developed for alcohol sulfotransferase (AST). This was the first continuous fluorometric assay reported for AST. It used 3'-phosphoadenosine 5'-phosphosulfate regenerated from 3-phosphoadenosine 5'-phosphate by a recombinant phenol sulfotransferase (PST) using 4-methylumbelliferyl sulfate as the sulfuryl group donor. The recombinant PST did not use the alcohol substrate under the designed condition, and the sensitivity for AST activity was found to be comparable to that of radioactive assay as reported in the literature. The change of fluorescence intensity of 4-methylumbelliferone corresponded directly to the amount of active AST and was sensitive enough to measure nanogram or picomole amounts of the enzyme activity. This fluorometric assay was used to determine the activities of AST as purified form and in crude extracts of pig liver, rat liver, and Escherichia coli. Some properties of human dehydroepiandrosterone sulfotransferase were determined by this method and were found to be comparable to published data. Under similar assay conditions, the contaminated activities of arylsulfatase in crude extracts were also determined. This method not only is useful for the routine and detailed kinetic study of this important class of enzymes but also has the potential for the development of a high-throughput procedure using microplate reader.

Spontaneous fetal loss caused by placental thrombosis in estrogen sulfotransferase-deficient mice

Estrogen sulfotransferase (EST, encoded by SULT1E1) catalyzes the sulfoconjugation and inactivation of estrogens. Despite decades of biochemical study and the recognition that high levels of estrogen sulfates circulate in the blood of pregnant and nonpregnant women, the physiological role of estrogen sulfation remains poorly understood. Here we show that ablation of the mouse Sult1e1 gene caused placental thrombosis and spontaneous fetal loss. This phenotype was associated with elevated free estrogen levels systemically and in the amniotic fluid, increased tissue factor expression in the placenta and heightened platelet sensitivity to agonist-induced activation ex vivo. Treatment of pregnant Sult1e1-null mice with either an anticoagulant or antiestrogen prevented the fetal loss phenotype. Our results thus identify Est as a critical estrogen modulator in the placenta and suggest a link between estrogen excess and thrombotic fetal loss. These findings may have implications for understanding and treating human pregnancy failure and intrauterine growth retardation.

Aberrant cholesterol transport and impaired steroidogenesis in Leydig cells lacking estrogen sulfotransferase

Estrogen sulfotransferase (EST) is a cytosolic enzyme that catalyzes the sulfoconjugation and inactivation of estrogens. It is expressed abundantly in the mammalian testes in which it may modulate the activity of locally produced estrogen. We demonstrate here that testicular Leydig cells from mice rendered deficient in EST expression by targeted gene deletion acquire a phenotype of increased cholesterol ester accumulation and impaired steroidogenesis with natural aging or in response to estrogen challenge. Abnormal accumulation of cholesterol ester in the mutant Leydig cells correlated with induced expression of the scavenger receptor type B class I, and cultured EST-deficient but not wild-type Leydig cells avidly uptook high-density lipoprotein cholesterol ester ex vivo. EST-deficient Leydig cells in culture produced 50-70% less testosterone than wild-type cells. This deficiency was reversed by androstenedione but not progesterone supplementation, indicating that reduced activities of 17-alpha-hydroxylase-17, 20-lyase were responsible. This conclusion was corroborated by decreased expression levels of 17-alpha-hydroxylase-17, 20-lyase but not of other key steroidogenic enzymes in the mutant cells. These results suggest that EST plays a physiologic role in protecting Leydig cells from estrogen-induced biochemical lesions and provide an example of critical regulation of tissue estrogen sensitivity by a ligand-transformation enzyme rather than through estrogen receptors.

Crystal structure of the human estrogen sulfotransferase-PAPS complex: evidence for catalytic role of Ser137 in the sulfuryl transfer reaction

Estrogen sulfotransferase (EST) transfers the sulfate group from 3'-phosphoadenosine 5'-phosphosulfate (PAPS) to estrogenic steroids. Here we report the crystal structure of human EST (hEST) in the context of the V269E mutant-PAPS complex, which is the first structure containing the active sulfate donor for any sulfotransferase. Superimposing this structure with the crystal structure of hEST in complex with the donor product 3'-phosphoadenosine 5'-phosphate (PAP) and the acceptor substrate 17beta-estradiol, the ternary structure with the PAPS and estradiol molecule, is modeled. These structures have now provided a more complete view of the S(N)2-like in-line displacement reaction catalyzed by sulfotransferases. In the PAPS-bound structure, the side chain nitrogen of the catalytic Lys(47) interacts with the side chain hydroxyl of Ser(137) and not with the bridging oxygen between the 5'-phosphate and sulfate groups of the PAPS molecule as is seen in the PAP-bound structures. This conformational change of the side chain nitrogen indicates that the interaction of Lys(47) with Ser(137) may regulate PAPS hydrolysis in the absences of an acceptor substrate. Supporting the structural data, the mutations of Ser(137) to cysteine and alanine decrease gradually k(cat) for PAPS hydrolysis and transfer activity. Thus, Ser(137) appears to play an important role in regulating the side chain interaction of Lys(47) with the bridging oxygen between the 5'-phosphate and the sulfate of PAPS.

The dimerization motif of cytosolic sulfotransferases

Cytosolic sulfotransferases sulfate steroids such as estrogens and hydroxysteroids. The enzymes, including human estrogen sulfotransferase (hEST) and hydroxysteroid sulfotransferase (hHST), are generally homodimers in solution with mouse estrogen sulfotransferase (mEST) being one of few exceptions. To identify the amino acid residues responsible for the dimerization, eight residues on the surface of hEST were mutated to their counterparts in mEST and mutated hESTs were then analyzed by gel filtration chromatography. A single mutation of Val(269) to Glu was sufficient to convert hEST to a monomer and the corresponding mutation of Val(260) also altered hHST to a monomer. The hHST crystal structure revealed a short stretch of peptide with the side-chains from two hHST monomers forming a hydrophobic zipper-like structure enforced by ion pairs at both ends. This peptide consisted of 10 residues near the C-terminus that, including the critical Val residue, is conserved as KXXXTVXXXE in nearly all cytosolic sulfotransferases. When mEST underwent the double mutations Pro269Thr/Glu270Val dimerization resulted. Thus, the KXXXTVXXXE sequence appears to be the common protein-protein interaction motif that mediates the homo- as well as heterodimerization of cytosolic sulfotransferases.

Bacterial expression, purification, and characterization of rat hydroxysteroid sulfotransferase STa

Hydroxysteroid (alcohol) sulfotransferase catalyzes numerous reactions that are important to our understanding of the metabolism of both endogenous steroids and exogenous alcohols. Here we report a method for prokaryotic expression and rapid purification of the recombinant hydroxysteroid sulfotransferase STa, a major isoform of hydroxysteroid sulfotransferase in the rat. The cDNA encoding STa was cloned into a pET-3c vector and expressed in Escherichia coli BL21 cells. After disruption of the cells by sonication, the enzyme was purified in one step by affinity chromatography on adenosine 3',5'-diphosphate-agarose. The purified recombinant STa had a relative molecular mass on SDS-PAGE that was identical with the native hepatic STa in rat liver. The expressed enzyme displayed similar substrate inhibition characteristics with dehydroepiandrosterone as have been noted previously with the native enzyme purified from rat liver. Furthermore, the catalytic efficiency in sulfation of 7-hydroxymethyl-12-methylbenz[a]anthracene, as well as the stereoselectivity of sulfation of the enantiomers of 1-phenyl-1-heptanol and 1-naphthyl-1-ethanol, catalyzed by the recombinant STa were consistent with characteristics of the STa isolated from rat liver.

Structure-function modeling of the interactions of N-alkyl-N-hydroxyanilines with rat hepatic aryl sulfotransferase IV

Although previous investigations have clearly shown that N-hydroxy arylamines and N-hydroxy heterocyclic amines are substrates for sulfotransferases, relatively little is known about which structural features of the N-hydroxy arylamines are important for sulfation to occur. The purpose of this investigation was to determine the extent to which secondary N-alkyl-N-hydroxy arylamines interact with aryl sulfotransferase (AST) IV (also known as tyrosine-ester sulfotransferase or ST1A1) and to evaluate these interactions using molecular modeling techniques. AST IV is a major cytosolic sulfotransferase in the rat, and it catalyzes the sulfation of various phenols, benzylic alcohols, arylhydroxamic acids, oximes, and primary N-hydroxy arylamines. In this study, three secondary N-hydroxy arylamines, N-hydroxy-N-methylaniline, N-ethyl-N-hydroxyaniline, and N-hydroxy-N-n-propylaniline, were found to be substrates for the purified rat hepatic AST IV. However, when the N-alkyl substituent was an n-butyl group (i.e., N-n-butyl-N-hydroxyaniline), the interaction with the enzyme changed from that of a substrate to competitive inhibition. This change in specificity was further explored through the construction and use of a model for AST IV based on mouse estrogen sulfotransferase, an enzyme whose crystal structure has been previously determined to high resolution. Molecular modeling techniques were used to dock each of the above N-hydroxy arylamines into the active site of the homology model of AST IV and determine optimum ligand geometries. The results of these experiments indicated that steric constraints on the orientation of binding of secondary N-alkyl-N-hydroxy arylamines at the active site of AST IV play a significant role in determining the nature of the interaction of the enzyme with these compounds.

Substrate gating confers steroid specificity to estrogen sulfotransferase

Estrogen sulfotransferase (EST) exhibits a high substrate specificity and catalytic efficiency toward estrogens such as estradiol (E2) but insignificant ability to sulfate hydroxysteroids such as dehydroepiandrosterone (DHEA). To provide the structural basis for this estrogen specificity, we mutated amino acid residues that constitute the substrate-binding site of EST. Among these mutants, only Tyr-81 decreased E2 and increased DHEA sulfotransferase activities. Substitution for Tyr-81 by smaller hydrophobic residues increased K(m(E2)) for E2 activity, whereas the k(cat(E2)) remained relatively constant. The Y81L mutant exhibited the same DHEA activity as wild-type hydroxysteroid sulfotransferase, for which K(m(DHEA)) remained relatively constant, and k(cat(DHEA)) was markedly increased. The side chain of Tyr-81 is directed at the A-ring of the E2 molecule in the substrate-binding pocket of EST, constituting a steric gate with Phe-142 sandwiching E2 from the opposite side. The present mutagenesis study indicates that the 3beta-hydroxyl group of the DHEA molecule is excluded from the catalytic site of EST through steric hindrance of Tyr-81 with the C-19 methyl group of DHEA. Thus, this stricture-like gating caused by steric hindrance appears to be a structural principle for conferring estrogen specificity to EST.

The sulfuryl transfer mechanism. Crystal structure of a vanadate complex of estrogen sulfotransferase and mutational analysis

Estrogen sulfotransferase (EST) catalyzes transfer of the 5'-sulfuryl group of adenosine 3'-phosphate 5'-phosphosulfate (PAPS) to the 3alpha-phenol group of estrogenic steroids such as estradiol (E2). The recent crystal structure of EST-adenosine 3', 5'-diphosphate (PAP)- E2 complex has revealed that residues Lys48, Thr45, Thr51, Thr52, Lys106, His108, and Try240 are in position to play a catalytic role in the sulfuryl transfer reaction of EST (Kakuta Y., Pedersen, L. G., Carter, C. W., Negishi, M., and Pedersen, L. C. (1997) Nat. Struct. Biol. 4, 904-908). Mutation of Lys48, Lys106, or His108 nearly abolishes EST activity, indicating that they play a critical role in catalysis. A present 2.2-A resolution structure of EST-PAP-vanadate complex indicates that the vanadate molecule adopts a trigonal bipyramidal geometry with its equatorial oxygens coordinated to these three residues. The apical positions of the vanadate molecule are occupied by a terminal oxygen of the 5'-phosphate of PAP (2.1 A) and a possible water molecule (2. 3 A). This water molecule superimposes well to the 3alpha-phenol group of E2 in the crystal structure of the EST.PAP.E2 complex. These structures are characteristic of the transition state for an in-line sulfuryl transfer reaction from PAPS to E2. Moreover, residues Lys48, Lys106, and His108 are found to be coordinated with the vanadate molecule at the transition state of EST.

A role of Lys614 in the sulfotransferase activity of human heparan sulfate N-deacetylase/N-sulfotransferase

An active sulfotransferase (ST, residues 558-882) domain of the human heparan sulfate N-deacetylase/N-sulfotransferase (hHSNST) has been identified by aligning the amino acid sequence of hHSNST to that of mouse estrogen sulfotransferase (EST). The bacterially expressed ST domain transfers the 5'-sulfuryl group of 3'-phosphoadenosine-5'-phosphosulfate (PAPS) to only deacetylated heparin with an efficiency similar to that previously reported for the purified rat HSNST. Moreover, the K(m,PAPS) (2.1 microM) of the ST domain is also similar to that of the rat enzyme. Lys48 is a key residue in mEST catalysis. The residue corresponding to Lys48 is conserved in all known heparan sulfate sulfotransferases (Lys614 in the ST domain of hHSNST). Mutation of Lys614 to Ala abolishes N-sulfotransferase activity, indicating an important catalytic role of Lys614 in the ST domain. Crystals of the ST domain have been grown (orthorhombic space group P2(1)2(1)2) with diffraction to 2.5 A resolution.